1. Field of the Invention
The present invention relates to partial response filters, and more particularly to a digital partial response filter which has particular application in an FM-SCA (Subsidiary Communications Authorization) digital data transmission system.
2. Description of the Prior Art
Partial response data transmission systems are known. In the known systems, for example, see U.S. Pat. Nos. 3,388,330 to Kretzmer, 3,492,578 to Gerrish and Re. 30,182 to Howson, binary or multilevel digital data is filtered by analog filtering means so that the transfer function of the entire system comprising transmitter, channel and receiver provides a partial response output in response to each input data symbol. By partial response output is meant that the output signal extends in a controlled way over more than one signalling interval, thus causing controlled intersymbol interference. Partial response filtering allows a doubling of the normally obtained signalling rate or, as is known from the work of Nyquist, the maximum symbol rate attainable over a channel of limited bandwidth, i.e., twice the bandwidth of the channel. Alternatively, partial response filtering allows a reduction in bandwidth, which is significant in view of the bandwidth limitations imposed on F.M. subcarrier use by the FCC. Although intersymbol interference is increased by partial response techniques, the interference is well defined and can be eliminated by known techniques. See, e.g., Principles of Digital Data Transmission, by A. P. Clark, 1976, pp. 154-192.
The known partial response filters perform their filtering function in an analog manner. It is more convenient, and more accurate data transmission is possible, if the digital data is processed digitally rather than by analog means, however.
It is therefore an object of the present invention to provide a digital realization of a partial response filter.
It is a further object of the present invention to provide a digital partial response filter which has particular application in an FM-SCA digital data transmission system wherein digital data is transmitted by frequency modulating an F.M. broadcast's station's SCA subcarrier.
It is a yet further object of the present invention to provide a digital partial response filter which allows for adaptation to meet the varying characteristics of different channels.